1. Field of the Invention
The present invention relates to a device and method for pre-stressing the cable system of a vehicle brake actuating system.
2. Brief Description of Related Art
After installing a brake system in a motor vehicle, it is often desirable to “pre-stress” the cables of the brake system to reduce the development of slack in the cables from load setting that may occur during the early life of the vehicle. For example, in a brake system that has not been pre-stressed and uses cables that are guided by flexible conduits extending between the actuator and the brakes, when the brakes are first applied, the tension in the cables tends to cause some straightening at their bends, which will in turn cause compression of the conduits. Compression of the conduits in turn leads to the cables developing an “apparent stretch.” This term describes the condition of the cable behaving as if it had become longer, when in fact its guide conduit has become shorter. In systems with no slack take-up mechanism, this presents the problem of losing cable tension relatively early in the vehicle's life. Even in systems with a slack take-up device, it is undesirable to have to waste the slack take-up capability on this development of apparent stretch and would be beneficial to preserve the slack take-up capability for taking up slack due to actual cable stretch that can occur over a much longer period of time.
One way to deal with this issue is to repeatedly operate the actuator to increase and decrease the tension in the cables prior to shipping the vehicle, thereby compressing the cable conduits. This pre-stressing reduces the development of subsequent compression of the cable conduits and the accompanying development of apparent cable stretch. However, for manually operated brake actuators, it is labor intensive to have a person repeatedly operate the actuator a sufficient number of times to effectively pre-stress the brake system (usually 10 or more times is desirable). Also, this type of work can lead to repetitive motion disorders for the worker assigned to this task.
U.S. Patent Application No. 2003/0227010A1 discloses one approach to pre-stressing the system. However, this approach is unsatisfactory because it requires a specialized tool that may not interface well with some vehicle designs.
Another approach to pre-stressing the system is disclosed in U.S. Pat. No. 5,983,745 and U.S. Patent Application No. 2002/011129A1. This approach works by using a shear tab in its cable connector. The shear tab is positioned so that as the actuator is operated, the tension applied to the cables is much higher than would normally be experienced, and ultimately the shear tab breaks. Then the cable tension is relaxed to a normal operational range. This approach is a “one-shot” approach, as the shear tab breaks on the first operation of the actuator, and relies on using an extra high amount of cable tension. This approach suffers the drawback that it does not replicate the repeated, cyclical type of operation that a brake system experiences during usage.
The present invention provides an improved device and method for pre-stressing the cable system of a vehicle brake actuating system.